UPFC MODELING, SIMULATION AND ITS EFFECT ON POWER SYSTEM PROTECTION

UPFC MODELING, SIMULATION AND ITS EFFECT ON POWER SYSTEM PROTECTION

Pankaj Khandelwal

, Bharat Modi

1 M.Tech Scholar Power System, S.K.I.T.Jaipur, (India)

2 Reader, Electrical Egineering, S.K.I.T.Jaipur, (India)

ABSTRACT

There is a continuously growing demand for wind power generation capacity. This situation forces the revision of the grid codes requirements, to remain connected during grid faults, i.e., to ride through the faults, and contribute to system stability during fault condition. In a typical fault condition, the voltage at the Point of Common Coupling (PCC) drops below 80% immediately and the rotor speed of induction generators becomes unstable. In this paper, Unified Power Flow Controller (UPFC) is used to improve the low voltage ride- through (LVRT) of wind energy conversion system (WECS) and to damp the rotor speed oscillations of induction generator under fault conditions. By controlling the UPFC as a virtual inductor, we aim to increase the voltage at the terminals of the wind energy conversion system (WECS) and thereby mitigate the destabilizing electrical torque and power during the fault. The DFIG-based WECS is considered for study here, equipped with a doubly fed induction generator (DFIG). The simulation results show that UPFC can improve the LVRT of DFIG-based WECS and hence maintaining wind turbine connection to the grid during certain levels of voltage fluctuation at the grid side.

Keywords:LVRT, Indian Electricity Grid code, UPFC, DFIG-WECS

1. INTRODUCTION

Recently non conventional energy sources are becoming very popular and as they are infinite and clean source of electricity [1,2]. Wind energy is most popular dominant source among renewable sources of energy [3].

Among the wind turbine concepts, turbines using the doubly fed induction generator (DFIG) are dominant due to its variable-speed operation, its separately controllable active and reactive power, and its partially rated power converter. But the reaction of DFIGs to grid voltage disturbances is sensitive, for symmetrical and unsymmetrical voltage dips, and requires additional compensation support to keep the voltage within area bounded by the LVRT and HVRT margins of the electricity grid codes. Fault-ride through (FRT) requirement is imposed on a wind power generator so that it remains stable and connected to the network during the network faults. Disconnection from grid may worsen the situation and can threaten the security standards at high wind penetration. The wind farm must be able to operate satisfactorily during and after the disturbances in the distribution/ transmission network, and remain connected to the grid without tripping from the grid for a specified period of time during a voltage drop (LVRT) or voltage swell (HVRT) at the PCC [5].

Flexible AC transmission system (FACTS) devices have been used to maintain the WTGs penetration to the electricity grid during fault conditions and wind speed variation. This work investigates the application of

unified power flow controller (UPFC) to improve the wind turbine FRT capability in compliance with Indian Electricity grid codes. FACTS devices are needed to which can either, compensate the voltage, phase shift, or both the increase of voltage and phase shift, and real and reactive power enhancement. Among various FACTS devices we have analyzed the performance of grid connected DFIG-WES system without and with UPFC as this custom power device has unique capability of series as well shunt compensation [6].

II. CASE STUDY: MATLAB SIMULATION AND ANALYSIS

2.1 System under Study: Fig.1 shows the system under study to analysis the effect of UPFC on stability of a power system with integrated wind energy generation system.

Fig.1 Single line diagram of system under study

Fig. 2 Simulink model of uncompensated system

Since our target is to study the effect of wind power integration into our existing power system so we have considered the other sources as ideal source which are capable of providing power to grid at constant frequency and voltage level. There are two ideal sources in the system each of which is of 120KV located at station 1 & 2 respectively away from each other and the two stations are connected via a line of 50 Km. A DFIG type wind generator of 12MW comprising of 2MW each of 6 units, with rated output voltage 575V supplies the grid connected to station 1 via 65 Km and to station 2 via 50 Km line through 25 kV distribution feeder system and exports power to a 120 kV grid. So the overall system forms a loop of 5 buses as shown in figure 1. The DFIG wind generation system consist of total 6 units each of 2MW and supplies a local resistive load of 500KW also a filter of 0.9 Mvar(Q=50)is connected at the 575 V wind generator bus. The ratings of various parameters of wind turbine have been described below. The single simulated DFIG type wind generator shown in fig. 1 actually consist of 6-wind-turbine farm but here its Simulink model have been obtained by multiplying the respective parameter of single turbine by 6, and parameters of single turbine are given below:

 Nominal mechanical output power of single wind turbine: 2×10⁶ watts,

 Rated DC bus capacitor per turbine: 10000 microfarads.

 Rated output power per generator: 2/0.9 MVA (at 0.9 PF) and

 Mode of operation: Voltage regulation.

With the enormous global growth in electrical power demand, there has been a challenge to deliver the required electrical power considering the quality sustainability and reliability of the delivered power. To achieve this goal, it is essential to control the existing transmission systems for efficient utilization and to avoid new costly installations [9]. FACTS technology play an important role in improving the utilization of the existing power system as it can provide technical solutions to improve the power system performance [10]. As a FACTS device, unified power flow controller allows power systems to be more flexible by using high-speed response and decoupled active and reactive power compensations and by installing UPFC at particular locations of the transmission system, the power dispatch can be increased up to the power rating of generators, transformers and thermal limits of line conductors, by increasing the stability margin. Shunt and series converters of the UPFC can control both active and reactive powers in four quadrants smoothly, rapidly and independently [11].

Fig. 3 UPFC configuration

Fig. 4 Simulink model of compensated system With UPFC.

For our analysis we have considered two position of unified power flow controller (UPFC). UPFC connected at the point of common coupling bus to gain high WTG damping and to provide reactive power absorption/generation support during fault conditions.

III. SIMULATION AND RESULT

3.1 Matlab simulation results of uncompensated system under study

The fig. 5 shows the output real and reactive power of wind farm at constant rated wind speed without compensation.

Fig. 5 PCC voltage, real and reactive power with constant wind speed from wind farm without compensation

From fig 5 it seems that operating power factor of wind farm is less than 0.95 leading.To study the effect of variation of wind speed we have simulated the test system to run initially at wind speed 15 m/s, thenat t = 50 s, wind speed is decreased to nearly half of its value to 8 m/s m/s as shown in fig 6

Fig 6. Initially wind speed 15 m/s, then at t = 50 s, wind speed is decreased to nearly half of its valueto8 m/s with wind farm without compensation

The following parameters signals are observed/monitored to study the effect of variation of wind speed on the

"Wind Turbine" scope and represented in fig. 7. Output generated active & reactive powers, ac output voltage, current, DC bus voltage and turbine speed. With initial wind speed of 15m/s the real power generation start increasing gradually with time and constant wind speed reaches to 0.9pu of rated real power in 18 seconds, but same time generated reactive power follows an inverse relation with real power. Generated reactive power initially is 0.58pu when real power is only 0.25pu initially. With increase in real power generation up to 0.9pu reactive power reduces to 0.5 pu. As per Indian electricity grid code for wind energy generation system interconnection a wind generator should operate with power factor limit of ±0.95 during steady state.

Fig. 7.Variation of output voltage, real and reactive power with variation of wind speed from wind generator.

In our case the power factor at start remains within limit of 0.65 to 0.89 leading. Even when system has achieved steady state at rated wind speed the power factor is cos (tan^-1(0.5/0.9)) = 0.874. Now this condition is violation of grid codes so reactive power generation is required to be compensated or wind system has to be pay penalty for not maintaining the pf. Again as wind speed falls to half of its rated value suddenly the real power generation reduces drastically and power factor is falls down below 0.24. Under these situations wind generator is not allowed to remain connected to grid as per Indian electricity grid codes.

3.1.2 Performance Under fault in120-kV Grid system

We now observe impact of a single phase-to-ground fault occurring on the 120-kV line atB120 bus. Now by opening the "Fault" block menu and selecting "Phase A Fault". We check that the fault is programmed to apply a 9-cycle single-phase to ground fault at t = 50s when the system has already achieved steady state.

Fig. 8 Power and voltage profile of PCC pre-fault, during and post fault conditions without compensation device.

From the above fig. 8 we observe that when a LG fault occur on the high voltage transmission system the wind turbine is in "Voltage regulation" mode, the positive-sequence voltage at wind-turbine terminals (V_B575) drops to 0.81pu during the fault, which is above the under voltage protection threshold (0.75 pu for a t > 0.1 s).

The wind farm therefore stays in service. But the real power support to the system from wind farm is zero during the fault. As per the IEGC wind farm must be able to support the grid during fault should generate real

power to system to increase transient stability of the system. As in case due to fault if some conventional unit is lost system will require spinning reserve support to cover the loss. So wind farm should be able to act as spinning reserve during fault if system voltage has not collapse i.e. fault ride through (FRT) capacity. Further during fault if under voltages condition occur wind generator require reactive power, which will be drawn by generators from grid if there is no reactive power support for generators. This will result in condition of over current and may cross the thermal limit of transmission line connecting the wind farm to grid. So it may be required disconnection of wind farm from grid. Thus, reducing the transient stability of overall system. The duration of FRT depends on magnitude of voltage drop at point of connection and time required to clear the grid fault.

3.1.2.1 Fault voltage and current profile at load bus away from wind farm

Fig. 9 (a) Current profile during fault (b) voltage profile during at far buses inuncompensated system

Fig. 9 show the voltage and current variation during fault in high voltage network. It shows the voltage and current variation at two buses namely at bus_121 and bus_124. Here we find that during fault voltage drops to nearly 0.6 in faulty phase, and fault current is 43.5pu.

3.3 Performance of System Compensated with UPFC

Without any compensation the system has the problem of operating at power factor which is out of permissible limit even operating at rated steady state input wind speed and output voltage at point of connection. Now, in system having UPFC as compensating device at point of common connection the performance are drawn in fig.

10.

Fig. 10 Graph showing Real and reactive power at PCC with variable wind speed with UPFC.

With and without compensation using UPFC voltage at the PCC is seems to be constant throughout the operation. At constant wind speed as well as during sudden transition of wind speed and after getting settled at lower wind speed the magnitude of voltage at common bus is unity. But without UPFC compensation the reactive power generation is quite large for below rated speed so wind farm is operating at leading pf less than 0.95. Even at rated speed the power factor is +0.91 leading which should be within ±0.95 the range of according to IEGC .Now, the system compensated with UPFC at common connection point the reactive power generation or absorption by DFIG is maintained zero whatsoever is the wind speed. So, wind generator operates at unity power factor at all speeds.

3.3.2 Performance under fault in 120-kV grid system

Simulation is carried out with a fault at the grid side that causes voltage sag at the PCC bus at t= 50 s for duration of 9 cycles of ac. The voltage performance at the point of common coupling is investigated during the fault without and with the connection of the UPFC to the PCC bus. The fig. 11 show the voltage, real and reactive power profile at PCC at pre-fault, during and post fault in grid system compensated with UPFC.

Fig. 11 Voltage, real and reactive power at PCC pre-fault, during and post-fault in UPFC compensated system.

As can be seen from the fig. 11 voltage at PCC drops to 0.76 p.u. during the LG fault in grid, but the real power support to the grid from wind farm does not falls to zero as happened in uncompensated system. During fault in grid real power support from the wind farm fallen to zero. But now when the system has been provided UPFC for compensation. The wind farm is providing the real power to grid thus, increase transient stability of the system. Fig. 12 and 13 shows the voltage across the DC-link capacitor of the WTG (V_DC) without and with the

UPFC compensation respectively. With the UPFC connected to the system, the overshooting and settling time are substantially reduced compared to the system without the connection of the UPFC.

Fig. 12 Voltage across the DC-link capacitor of the WTG at pre-fault, during –fault and post- fault in uncompensated system.

As can be seen the DC voltage overshoot in WTG is crossing the 1300V during faults and falls down to 1100V when no compensation is provided, as compared the maximum voltage attained is only 1250V and falls down to 1190V only in compensated system. Also WTG capacitor voltage is maintained to steady value within 0.5 seconds in compensated system as compared to uncompensated system.

Fig. 13 Voltage across the DC-link capacitor of the WTG at pre-fault, during –fault and post- fault in compensated system.

It worth to notice that during normal operating conditions, there is no reactive power exchange between the UPFC and the AC system and the reactive power generation is maintained at zero level to achieve unity power factor operation for the WTG.

3.3.2.1 Fault voltage and current profile at high voltage system from wind farm in compensated system

The figure 14 below shows the voltage and current profile during fault in compensated system. From fig. 9 It seems that during fault voltage drops nearly to 0.6 in faulty phase but the fault current reduces considerably.

Without UPFC fault current was 43.5 pu ampere but now with compensated system it maximum value found to be only 23 pu ampere. Thus, the UPFC not only make possible the real power generation from wind farm during fault but also reduces fault current and so fault level in high voltage system. These all effects increases the transient stability of the system overall.

Fig. 14 (a) Current profile during fault (b) voltage profile during at far buses incompensated system

IV.CONCLUSION

This work investigates the application of UPFC to enhance the FRT of wind energy conversion system to comply with the grid codes of Indian Electricity and US. Results show that, without UPFC, WTGs must be disconnected from the grid during voltage swell or voltage sag event to avoid the turbines from being damaged, as the voltage at the PCC will violate the safety margins required for both studied grid codes. The presence of UPFC can significantly improve the FRT capability of the WTGs and hence their connection to the grid can be maintained to support the grid during fault conditions and to guarantee the continuity of its power delivery to the grid.

V. ACKNOWLEDGMENT

It is the contribution of many persons that make a work successful. I wish to express my gratitude to individuals who have contributed their ideas, time and energy in this work. I wish to express my heartfelt gratitude to my supervisor Mr. Bharat Modi, Reader, Dept. of Electrical Engineering, SKIT, Jaipur, Rajasthan for his active interest, constructive guidance and valuable advice during every stage of this work. His guidance coupled with active and timely review of my work provided the necessary motivation for me to work on and successfully complete the dissertation.

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